U.S. Pat. No. 6,872,466 discloses a method of repairing a damaged sifting or filtering screen having an outer frame across which the filtering mesh is stretched. Within the frame a rectangular grid of ribs divides the filtering area into an orthogonal array of cells. Damage to the mesh is repaired by snap-fitting a plastics plug into the cell underlying the damaged area of the mesh. This repairs the screen by plugging the cell and preventing any particles passing through that cell. However, such known plugs suffer from the disadvantages that they do not always seal adequately against the material of the screen and they do not always snap properly into the desired location and can sometimes fall out of the cell into which they have been placed, in each case this allowing some particles to pass through the damaged cell.